Fix bug in code for avoiding dynamic initialization of dllimport globals

Fix bug in code for avoiding dynamic initialization of dllimport globals

When instantiating dllimport variables with dynamic initializers, don't
bail out of Sema::InstantiateVariableInitializer without calling
PopExpressionEvaluationContext().

This was causing a stale object to stay on the ExprEvalContexts stack,
causing subsequent calls to getCurrentMangleNumberContext() to fail,
resulting in incorrect numbering of static locals (and probably other
broken things).

llvm-svn: 211137